Fouling of water-using, or aqueous, processes and systems, particularly industrial aqueous systems, is an extremely serious problem. Fouling results from the settling out of suspended solids, build up of corrosion products, and growth of microbial masses. Fouling interferes with heat transfer, reduces the water flow, and also promotes severe corrosion under the deposits. Microbial deposits in an aqueous system often require treatment with both a biocide and a dispersant to remove such deposits from surfaces. Among the problematic microbial organisms are algae, which, since they generally require sunlight to grow, are found on open, exposed areas, such as cooling tower decks. Most algae grow in dense, fibrous mats that plug distribution piping and flumes, and portions of such mats break off and deposit at other locations in the system. Algae also are a nutrient for other undesirable organisms and algae mats provide areas for subsequent growth of anaerobic bacteria.
For industrial aqueous systems, control of microbial growth by heat or radiation is generally impractical and bulk water treatment with nonoxidizing biocides is often prohibitively expensive and environmentally unsuitable. Thus chemicals that in water provide hypochlorous and hypochlorite ions, for instance chlorine, are often the bulk water treatment of choice.
In recirculating cooling towers, algae growth on the open decks present a serious problem. Large volumes of water are employed in such systems, and the water generally has a relatively short half-life in the system, and thus treatment of the bulk water with even 100 ppm of chemical biocide requires a great amount of biocide. Further, in some recirculating cooling towers the treatment of the bulk water with an oxidizing biocide such as chlorine has little to no effect with respect to algae growth on the cooling tower deck. If the algae growth on the deck becomes heavy, portions of even killed algae mats may break off and foul the heat exchanger. In practice cooling tower decks generally need to be cleaned or scrubbed to remove unacceptable levels of algae periodically, which requires the tower to be shut down, adding greatly to the overall cost of operations. Typically a cooling tower will be shut down for algae removal from the deck several times a season, and some towers require such deck cleaning several times a month.
It would be desirable to provide a method of controlling algae growth at the site of the open decks of recirculating cooling towers, or similarly situated areas in other aqueous systems, that is more cost effective than present bulk water treatments, including bulk water treatment with chlorine. It would be desirable to provide such a method that avoids bulk water treatment, and concommitant problems that often arise by the addition of chemical components to the bulk water, while effectively controlling the algae growth on the decks. It would be desirable to provide such a method that has a high degree of effectiveness and thus minimizes the frequency of deck scrubbing to remove algae growth. It is an object of the present invention to provide a method that has the above enumerated advantages and other advantages as described in more detail below.